unsigned long myTimer = 0;
const unsigned long RunPeriod = 2800;

unsigned long myCounter = 0;

boolean startHasBeenPressed = false;

const byte startButtonPin = 4;
const byte STOPButtonPin  = 5;

#define pressed  LOW
#define released HIGH

void setup() {
  Serial.begin(115200);
  Serial.println("Setup-Start");
  Serial.println("press green button to start non-blocking");
  Serial.println("time-based upcounting loop");
  Serial.println("press red button to STOP the 'looping'");

  pinMode(startButtonPin, INPUT_PULLUP);
  pinMode(STOPButtonPin,  INPUT_PULLUP);
}


void loop() {

  // check if upcounting is running
  if (startHasBeenPressed == true) {
    // check if STOP-button us pressed
    if ( digitalRead(STOPButtonPin) == pressed) {
      startHasBeenPressed = false;
      Serial.print("STOP-Button terminated upcounting at counter=");
      Serial.println(myCounter);
      myCounter = 0;

      Serial.print("time between pressing Start / STOP-Button ");
      Serial.print(millis() - myTimer);
      Serial.println(" milliseconds");
    }
  }

  // check if counting up through pressing the button
  // has not yet started
  if (startHasBeenPressed == false) {
    // check for button-press
    if ( digitalRead(startButtonPin) == pressed) {
      startHasBeenPressed = true;
      myTimer = millis(); // store snapshot of time
      Serial.println("Startbutton pressed start counting ....");
    }
  }

  // check if startbutton has been pressed
  if (startHasBeenPressed == true) {
    myCounter++;

    // check if number of milliseconds 
    // stored in variable RunPeriod have passed by
    if ( TimePeriodIsOver(myTimer, RunPeriod) ) {
      // when number of milliseconds stored in variable RunPeriod
      // have REALLY passed by
      startHasBeenPressed = false; // stop counting
      Serial.print("counting up for ");
      Serial.print(RunPeriod);
      Serial.print(" milliseconds is over myCounter=");
      Serial.println(myCounter);
      myCounter = 0; // reset counter to zero
    }
  }
}

// easy to use helper-function for non-blocking timing
boolean TimePeriodIsOver (unsigned long &startOfPeriod, unsigned long TimePeriod) {
  unsigned long currentMillis  = millis();
  if ( currentMillis - startOfPeriod >= TimePeriod ) {
    // more time than TimePeriod has elapsed since last time if-condition was true
    startOfPeriod = currentMillis; // a new period starts right here so set new starttime
    return true;
  }
  else return false;            // actual TimePeriod is NOT yet over
}